Implantable cardiac stimulator with electrode-tissue interface characterization

Abstract

A cardiac stimulator capable of measuring pacing impedance includes a tank capacitor for delivering charge to the heart via device leads, a shunt resistor, and high-impedance buffers for measuring pacing current through the shunt resistor. Soon after the leading edge of the stimulation pulse, the voltage across the shunt resistor, as sampled by a high-impedance buffer, indicates lead and cardiac tissue resistance. Just prior to opening the pacing switch to terminate the stimulation pulse, the voltage across the shunt resistor is sampled by a high-impedance buffer and held once again to allow the capacitance of the lead/heart tissue to be calculated. In alternative embodiments, a high-impedance buffer measures the voltage between the tank capacitor and ground immediately following the stimulation pulse to allow estimation of the lead/heart tissue capacitance. In one alternative embodiment, a look-up table is created in main memory and searched to find the closest lead/heart tissue capacitance estimate to any arbitrary degree of accuracy. In another alternative embodiment, the lead/heart tissue capacitance is estimated by successive approximation to any arbitrary degree of accuracy. When the lead/heart tissue capacitance and lead resistance have been determined, a plurality of parameters of importance for analyzing and optimizing a cardiac stimulation system may be calculated, such as the instantaneous current, the average current, the charge, and the energy delivered to the cardiac tissue.

What is claimed is: 
     
       1. A method, comprising: determining a lead/tissue resistance; and   using lead/tissue resistance to determine a Helmholtz capacitance of a patient's heart.   
     
     
       2. The method of claim 1 wherein determining a lead/tissue resistance comprises: applying a voltage V i  to a closed-loop current path comprising a lead, heart tissue, and components of known impedance;   measuring a voltage produced in the closed-loop current path by the voltage V 1  after applying the voltage V 1  to closed-loop current path, thereby producing a measured voltage; and   calculating the lead/tissue resistance based on the measured voltage.   
     
     
       3. The method of claim 2 wherein measuring a voltage comprises measuring a voltage across a resistor, thereby producing the measured voltage across the resistor. 
     
     
       4. The method of claim 3 in which the lead/tissue resistance is calculated using a formula which comprises the measured voltage across the resistor as a variable of the formula. 
     
     
       5. The method of claim 1 wherein using the lead/tissue resistance to determine a Helmholtz capacitance of a patient's heart further comprises: applying a voltage V i  for a fixed amount of time, T PW  seconds, to a closed-loop current path comprising a lead, heart tissue, and components of known impedance;   measuring a voltage produced in the closed-loop current path by the voltage V; at a time t=T PW  the time t=T PW  occurring just before time t=T PW , thereby producing a measured voltage; and   calculating the Helmholtz capacitance based on the measured voltage.   
     
     
       6. The method of claim 5 wherein the Helmholtz capacitance is calculated using a formula which comprises the measured voltage as a variable of the formula. 
     
     
       7. The method of claim 2 wherein using the lead/tissue resistance to determine a Helmholtz capacitance of a patient's heart further comprises: discharging a tank capacitor for a duration of T PW  seconds through a closed-loop current path comprising a lead, heart tissue, and components of known impedance;   measuring a voltage across the tank capacitor at a time t=T PW   +  just after the end of the duration of T PW  seconds, thereby producing a measured voltage; and   estimating the Helmholtz capacitance based on the measured voltage.   
     
     
       8. The method of claim 7 wherein estimating the Helmholtz capacitance includes calculating an empirical voltage value across the tank capacitor based on a predetermined empirical estimate of a Helmholtz capacitance C L  (empirical). 
     
     
       9. The method of claim 8 wherein estimating the Helmholtz capacitance further includes calculating additional empirical voltage values across the tank capacitor based on additional predetermined empirical estimates of the Helmholtz capacitance C L  (empirical). 
     
     
       10. The method of claim 9 wherein estimating the Helmholtz capacitance further includes estimating the Helmholtz capacitance to be the empirical estimate C L  (empirical) which is associated with the empirical voltage value that is closest to the measured voltage. 
     
     
       11. The method of claim 7 wherein estimating the Helmholtz capacitance comprises an iterative process of calculating an empirical voltage value across the tank capacitor based on a predetermined empirical estimate of a Helmholtz capacitance C L  empirical), calculating an approximation of a Helmholtz capacitance C L  (approx) based on the empirical voltage value, comparing C L  approx) with C L  (empirical) and adjusting C L  (empirical) for a subsequent iteration of the process, wherein the process is repeated until comparing C L  (approx) with C L  (empirical) indicates that C L  (approx) is within a predetermined limit of C L  (empirical). 
     
     
       12. The method of claim 11 wherein adjusting C L  (empirical) comprises adjusting C L  (empirical) by an amount equal to the predetermined limit.